1. Field of the Invention
The present invention relates to a reclosing system for a power transmission line, in which the arc ion extinguishing time of an accident phase is detected by a simple method to execute reclosing.
2. Description of the Related Art
There is a reclosing system of a direct grounding type for power transmission lines. As the reclosing system of this type, a single-phase reclosing system in which when there is one accident phase, only the accident phase is opened and reclosed, or a three-phase reclosing system in which when there are two accident phases, three phases are opened and reclosed, is employed. Further, there is a parallel-type double-line reclosing system. As the parallel double-line reclosing system, a multiphase reclosing system, in which an accident phase is opened, and is reclosed if two or three normal phases are still left, is employed for stable operation.
In general, the time ranging from the opening of a circuit breaker to its reclosing is called a no-voltage time period. The no-voltage time period is determined based on a supposed time period ranging from the cutoff of an accident current by the circuit breaker, to the arc-ion extinguished time point at the place of the accident. The no-voltage time period varies depending upon the state of the accident. In the single-phase and multiphase reclosing systems, the no-voltage time period is set to about 1 sec, while in the three-phase reclosing system, it is set to about 0.5 to 0.8 sec.
FIG. 19 is a time chart illustrating the operation performed from the opening of a circuit breaker CB by a transmission line protection relay RY to the reclosing of the breaker by the relay after a no-voltage time elapses, when an accident F due to, for example, ground discharge occurs in a single-line power transmission line TL. (a), (b) and (c) of FIG. 19 show the operation of a power system, the response of the circuit breaker CB, and the response of the protection relay RY, respectively.
As shown in (a), (b) and (c) of FIG. 19, the accident F occurs at time point t1 due to, for example, ground discharge, an accident current flows into the transmission line protection relay RY to operate the same, the relay RY outputs a cutoff command to the circuit breaker CB at time point t2, and the circuit breaker CB is opened at time point t3 to extinguish the accident current.
At the place of the accident F, the ambient air is ionized by arc discharge. If power transmission is resumed before arc ions are diffused and extinguished, arc current again flows and reclosing fails. In view of this, time point t4 (<t5) at which a reclosing command is issued is set so that the circuit breaker CB is reclosed at time point t5 assumed after a time period within which the arc ions are generally supposed to be completely extinguished elapses.
The time period (ion extinguishing time period) required until the arc ions are extinguished after the accident current is cut off varies depending upon primary arc current, system voltage, line constant length (on which line distributed capacity depends), wind velocity, etc. At any rate, it is necessary to set the no-voltage time period up to reclosing slightly longer than the ion extinguishing time period.
In general, the dielectric recovery time period and the ion extinguishing time period are longer as the accident current is greater and in proportion to the system voltage. The no-voltage time period for reclosing is set to about 1.0 sec. in the case of a 500 kV system, and to about 0.5 to 0.8 sec. in the case of a 187 to 275 kV system.
FIG. 20 is a graph showing the relationship between the nominal voltage and the time period required for ion extinguishing after cutoff due to an accident, which is assumed where the accident current is about 20 kA. At the accident place F, the air is ionized by arc discharge, and residual ions are left even after the accident current is cut off. The time period within which the residual ions are extinguished, i.e., the ion extinguishing time period, is deeply associated with the system voltage. Namely, the higher the system voltage, the longer the ion extinguishing time period.
For the power system, the no-voltage time period for reclosing significantly influences its stability. Therefore, it is preferable to minimize the no-voltage time period. To this end, Japanese Patent No. 3710771, for example, has proposed a technique of detecting the time when the ratio of harmonics in the line voltage of an accident cutoff phase is reduced (this time period corresponding to the arc-ion extinguishing time period), and adaptively (freely) changing the no-voltage time period based on the detection result.